01 April 2003
Signal conditioning can strengthen measurement data.
By Davis Mathews
Temperature measurement, which is a vital part of most industrial and process operations, is typically accomplished by a temperature sensor—a thermocouple or a resistance temperature detector (RTD)—in contact with a solid surface or immersed in a fluid.
Temperature sensors defined
While it may be second nature to most, the following are definitions for a thermocouple and a resistance temperature detector (RTD).
A thermocouple is a device for measuring temperature consisting of a pair of wires of different metals or semiconductors joined at both ends. One junction is at the temperature area you are looking to measure, the second at a fixed temperature. The electromotive force generated depends on the temperature difference.
Of the huge amount of possible metal combinations, ISA—The Instrumentation, Systems, and Automation Society recognizes 12. Most of these thermocouple types are known by a single-letter designation. The most common are J, K, T, and E. Thermocouple compositions are international standards, but the color codes of their wires are different. In the U.S., the negative lead is always red, while the rest of the world uses red to designate the positive lead.
An RTD is a variable resistor that will change its electrical resistance in direct proportion to changes in temperature in a precise, repeatable, and nearly linear manner.
Platinum, nickel, and copper are the most common materials that make up most RTDs. Copper and nickel versions operate at lower temperature ranges and are less expensive. Platinum is the most versatile material because of its wide temperature range (–200° to 850°C), excellent repeatability, stability, and resistance to chemicals and corrosion. Platinum RTDs are available in 100 (omega), 200 (omega), 500 (omega), and 1,000 (omega) nominal resistance values at 0°C, of which the 100 (omega) is the most popular.
But what if the device that reads the temperature sensor is several hundred feet away from the sensor? In this case, a user can turn to temperature transmitters.
The advantages and advances of conditioning the raw signal from the temperature sensor in the form of analog or digital format using temperature transmitters are clear.
In applications where a temperature sensor signal does not have to travel far, engineers often connect the temperature sensor directly to the monitoring or controlling device. There are several advantages to a direct connect solution, including simplicity.
There are also several disadvantages to a direct connect solution. One problem is that the sensor signal is more susceptible to being degraded or corrupted during the transmission process. This can happen as a result of the distance involved because temperature sensor signals are typically weak. It can also happen due to interference from other signals in the area of the wires doing the transmitting. Using a transmitter is a safeguard against signal corruption and degradation.
The cost of the connecting wire can be substantial in both material and labor costs. If the sensor is a thermocouple, the transmitting wire must match the type of the thermocouple. This might not be an issue if the thermocouple is only a few feet away. But if the signal has to travel any distance, then the wire and labor costs can be significant.
These factors should weigh against the cost and installation of a temperature transmitter.
Once a user has selected the temperature sensor, he must integrate it into the control system, usually based on a DCS or a PLC.
One method of integration is to directly connect the RTD or thermocouple lead wires to the controller. This technique requires a dedicated temperature conversion card. Multichannel temperature cards are available for different controllers but are expensive and do not offer system flexibility.
The first instrument in line after the sensor is often some type of signal conditioning instrument. Most often this device is a temperature transmitter. Temperature transmitters convert the signal produced by the sensor to an electrical signal recognizable to the processing instrumentation.
There are two basic types of temperature transmitters: four wire and two wire. They range from the simple, low-cost, analog dedicated transmitters to the more sophisticated, programmable/universal smart devices with hazardous area (Class 1, Div 1 or Div 2) ratings.
Four-wire transmitters use a power input that is separate from the signal transmitting wiring.
Two-wire transmitters use a DC power supply that supplies power to the transmitter over the same two wires used to transmit the signal. A two-wire transmitter draws current from a remote DC power supply, allowing the signal and power to combine (one circuit serves as two functions).
The main advantage of using two-wire transmitters is that you do not need AC power or a separate power source at the remote location, saving wiring time and labor costs.
Use of temperature transmitters is now becoming widespread in the industry, due to the added benefits they offer, including accuracy, isolation and noise immunity, system flexibility and diagnostics, and cost savings.
|Making a connection|
Temperature transmitters maintain the integrity of the sensor's output. Their accuracy specifications surpass those of a PLC or DCS card.
Conditioning the sensor signal near the measuring point prevents degradation of the signal from errors introduced by thermal gradients with thermocouples and resistance imbalance with RTD wires.
Transmitters also provide low-pass filtering that prevents high-frequency noise from passing through to the controller. The majority of head-mounting temperature transmitters are nonisolated, which means there is an electrical connection between the sensing element and the 4–20 mA loop.
In measurement applications where several transmitters work with one receiving instrument such as a PLC, isolated circuits are essential. If grounded tip thermocouples or other exposed sensors connect with a single power supply, then isolation is probably a must.
However, it could be less expensive to look at three alternatives to buying an isolated transmitter:
1) Replace the sensors with insulated types.
2) Use individual power supplies for each loop.
3) Use a separate isolating module mounted in the control panel.
(Some ceramic type thermocouple sheaths become electrically conducting at high temperatures, giving intermittent ground loop problems on multiple installations.)
|In the loop|
Signal conditioners provide the control system with complete flexibility. A 4–20 mA measurement signal can go directly to a recorder or the analog card. Some sophisticated temperature conditioners provide analog and digital outputs for alarming or emergency shutdown. The modules also provide local and remote indications in case of wire break.
Thermocouple or RTD wires connected directly to temperature cards are expensive, especially when labor, maintenance, and troubleshooting go into the system cost. The use of temperature transmitters in conjunction with standard analog and/or digital input cards can considerably reduce the cost.
Advances in surface mount technology and new component designs have allowed engineers to reduce the overall size. This allows the dimension of a transmitter to be the same as that of a connecting block, enabling the transmitter and the connecting block to fit within a standard temperature probe housing.
Modern digital "smart" transmitters offer distinct advantages for suppliers and users in terms of performance and product flexibility.
A user can easily and quickly configure one basic unit via a PC for different sensor types, operating range, and damping parameters—factors that significantly reduce stockholding requirements and ensure better delivery.
The use of HART (highway addressable remote transducer)-compatible devices has increased during the past decade. HART is a communications protocol used to "talk" to field devices digitally, while at the same time using the industry-standard 4–20 mA analog transmission.
Therefore, HART provides bidirectional data interchange while maintaining compatibility with existing 4–20 mA systems. It is widely used in process industries such as chemical, oil refining, pulp and paper, food, and pharmaceuticals.
Temperature transmitters have retained their 4–20 mA output across the range of designs. This faithful and reliable output has withstood the test of time throughout dynamic development changes.
With digital devices, it is now possible to define the failure mode as either "high" (upscale) or "low" (downscale) status. Temperature transmitters, especially head mounting types and compact DIN-rail types, are becoming more popular and accepted and should be carefully considered. TT
Behind the byline
Davis Mathews is product manager of signal conditioners at Phoenix Contact Inc. in Harrisburg, Pa. His e-mail address is firstname.lastname@example.org.